US20100134877A1 - Semiconductor optical amplifier with a reduced noise figure - Google Patents
Semiconductor optical amplifier with a reduced noise figure Download PDFInfo
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- US20100134877A1 US20100134877A1 US12/626,373 US62637309A US2010134877A1 US 20100134877 A1 US20100134877 A1 US 20100134877A1 US 62637309 A US62637309 A US 62637309A US 2010134877 A1 US2010134877 A1 US 2010134877A1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/50—Amplifier structures not provided for in groups H01S5/02 - H01S5/30
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
- H01S3/1301—Stabilisation of laser output parameters, e.g. frequency or amplitude in optical amplifiers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/02—Structural details or components not essential to laser action
- H01S5/026—Monolithically integrated components, e.g. waveguides, monitoring photo-detectors, drivers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S2301/00—Functional characteristics
- H01S2301/02—ASE (amplified spontaneous emission), noise; Reduction thereof
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- H—ELECTRICITY
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/0014—Measuring characteristics or properties thereof
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
- H01S5/042—Electrical excitation ; Circuits therefor
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
- H01S5/042—Electrical excitation ; Circuits therefor
- H01S5/0425—Electrodes, e.g. characterised by the structure
- H01S5/04256—Electrodes, e.g. characterised by the structure characterised by the configuration
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/12—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
- H01S5/1234—Actively induced grating, e.g. acoustically or electrically induced
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/12—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
- H01S5/125—Distributed Bragg reflector [DBR] lasers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/40—Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
- H01S5/4025—Array arrangements, e.g. constituted by discrete laser diodes or laser bar
- H01S5/4031—Edge-emitting structures
- H01S5/4056—Edge-emitting structures emitting light in more than one direction
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/50—Amplifier structures not provided for in groups H01S5/02 - H01S5/30
- H01S5/5063—Amplifier structures not provided for in groups H01S5/02 - H01S5/30 operating above threshold
- H01S5/5072—Gain clamping, i.e. stabilisation by saturation using a further mode or frequency
Definitions
- the present invention relates to a semiconductor optical amplifier (SOA) with a reduced noise figure.
- SOA semiconductor optical amplifier
- the present invention more particularly relates to a semiconductor optical amplifier with a control arrangement for selectively varying the carrier density along the amplification path thereby selectively controlling the amplified spontaneous emission (ASE) and consequently the noise figure.
- SOA Semiconductor Optical Amplifiers
- EDFA Erbium Doped fibre amplifier
- EDWA Erbium Doped waveguide amplifier
- Raman amplifier the SOA has many advantages: lower cost, larger bandwidth, smaller size, and the potential to be integrated on a chip with electrical pumping.
- SOAs suffer from the disadvantage that they have a higher noise figure than EDFA or EDWA amplifiers.
- a first embodiment of the invention provides a semiconductor optical amplifier as detailed in claim 1 .
- the invention also relates to a processing element as detailed in claim 42 .
- the invention relates to an electronic chip as detailed in claim 43 .
- Advantageous embodiments are provided in the dependent claims.
- FIG. 1 is a perspective view of a semiconductor optical amplifier (SOA) in accordance with the present invention.
- SOA semiconductor optical amplifier
- FIG. 2 is a cross sectional plan view of the SOA of FIG. 1 .
- FIG. 3 is a cross sectional side view of the SOA of FIG. 1
- FIG. 4 shows the SOA with the active medium extending into the mirrors.
- FIG. 5 shows a graph of the noise figure of the SOA of FIG. 1 as a function of the percentage of the lasing portion for various values of pumping currents.
- FIG. 6 shows a graph of the noise figure of the SOA of FIG. 1 as a function of the input power for various values of lasing proportions.
- FIG. 7 shows the carrier density distribution as a function of position along the amplified path of the SOA of FIG. 1 for various values of pumping currents.
- FIG. 8 is a perspective view of another semiconductor optical amplifier (SOA) in accordance with the present invention.
- SOA semiconductor optical amplifiers
- a semiconductor optical amplifier (SOA) 100 for amplifying an optical signal 102 .
- the present inventors have realised that by providing the SOA 100 with a control arrangement for selectively varying the carrier density along an amplification path significantly reduces the amplified spontaneous emission (ASE) associated with the SOA 100 which in turn reduces the noise figure.
- the SOA 100 defines a longitudinal axis 105 and a transverse axis 106 which is orthogonal to the longitudinal axis 105 .
- a semiconductor active medium 107 is provided on the SOA 100 which defines a single mode amplification path for amplifying the optical signal 102 as the optical signal 102 propagates along the amplification path.
- the active medium 107 has substantially uniform dimensions and is arranged to be coaxial with the longitudinal axis 105 .
- the active medium 107 extends between an input 110 and a spaced apart output 113 which accommodates the optical signal 102 there through.
- the active medium 107 is an elongated strip and is encapsulated (surrounded) by cladding 118 .
- the cladding 118 comprises a p-type cladding layer 122 and an n-type cladding type layer 125 which together encapsulate the active medium 107 .
- the p-type cladding layer 122 and the n-type cladding layer 125 together with the active medium form an SOA junction.
- the active medium 107 may comprises any suitable amplification material such as but not limited to Quantum wells, Multiple Quantum Wells, bulk material, q-dot, q-dash.
- suitable amplification material such as but not limited to Quantum wells, Multiple Quantum Wells, bulk material, q-dot, q-dash.
- Semiconductor active materials are well known to those skilled in the art, and is not intended to describe the active medium further.
- the control means comprises a lasing cavity 130 located on the cladding 118 for providing lateral lasing conditions in the amplification path towards the output 113 .
- the lasing cavity 130 can be considered to be enclosed by the dashed line 131 in FIG. 3 .
- the lasing cavity 130 extends from the output 113 and terminates spaced apart from the input 110 .
- the active medium 107 along the length of the lasing cavity 130 forms part of the lasing cavity 130 .
- the lasing cavity 130 includes the amplification path towards the output 113 but not the amplification path towards the input 110 as the lasing cavity 130 does not extend the full length of the active medium 107 .
- the lasing cavity 130 is of length which corresponds to approximately 60% of the length of the active medium 107 , as illustrated best in FIG. 2 . Therefore approximately 40% of the length of the active medium is not included in the lasing cavity 130 .
- the lasing cavity 130 divides the amplification path into two distinct portions, namely, a lasing portion 133 which forms part of the lasing cavity 130 and a non-lasing portion 135 which is not part of the lasing cavity 130 .
- the size of the lasing and non-lasing portions may vary depending on the application to which the SOA 100 is applied. The precise values of 40% and 60% are given by way of example only, and it is not intended to limit the SOA 100 to these precise values.
- the lasing cavity 130 is arranged for facilitating transverse lasing with respect to the direction of the optical signal 102 as the optical signal 102 propagates along the lasing portion 133 of the amplification path.
- lasing occurs in the lasing cavity 130 it sets the carrier distribution in the lasing portion 133 to a predetermined gain value. It will be appreciated by those skilled in the art that the gain in the non-lasing portion 135 remains substantially unaffected by lasing conditions in the lasing cavity 130 .
- the lasing cavity 130 also comprises a pair of highly reflective mirrors 140 located on respective opposite sides of the active medium 107 . Lateral cavities 144 are formed on the cladding 118 for accommodating the respective mirrors 140 therein. While FIG. 1 shows the mirrors 140 of uniform dimensions, in an alternative arrangement, the dimensions of the mirrors 140 progressively increase along the lasing portion 133 towards the output 113 such that the mirrors 140 define a stair case arrangement when viewed in plan view. In this arrangement the steps of the mirrors 140 may be provided as discrete segments.
- the mirrors 140 may be any suitable type such as Bragg reflectors. It is not intended to limit the mirrors 140 to any particular configuration or type, Bragg reflectors are given by way of example only.
- the mirrors 140 have been shown to be spaced apart from the active medium 107 , the present inventors envisage that the active medium 107 may be extended into the mirrors 140 as illustrated in FIG. 4 .
- the mirrors 140 in the exemplary arrangement consist of air and cladding material. However, it will be appreciated by those skilled in the art that the mirrors 140 may be formed from a mix of dielectric materials.
- the p-type cladding layer 122 defines a platform in the form of a ridge 150 , in this case, of rectangular cross sectional area on which an electrical contact 155 is supported for facilitating pumping the active medium 107 with current, as best illustrated in FIG. 1 .
- the electrical contact 155 is substantially the same width as the active medium 107 .
- the electrical contact 155 defines a longitudinal axis which is substantially coaxial with the longitudinal axis 105 of the SOA 100 .
- the electrical contact 155 is in registration with the active medium 107 and spaced apart therefrom by the P-type cladding layer 122 . It is not intended to limit the invention to the pumping arrangement described in the exemplary embodiment. It will be appreciated by those skilled in the art that alternative arrangements may be provided for pumping the active medium 107 with current.
- the SOA 100 receives the optical signal 102 to be amplified at the input 110 of the amplification path.
- the semiconductor active medium 107 amplifies the optical signal 102 such that an amplified version of the optical signal 102 is emitted from the output 113 of the amplification path.
- the semiconductor active medium 107 is pumped with current by applying electrical current to the electrical contact 155 .
- the pumping of the active medium 107 with current causes the P-type cladding layer 122 to inject holes into the active medium 107 , and the N-type cladding layer 125 to inject electrons into the active medium 107 resulting in the active medium 107 being pumped with carriers.
- the operation of SOA junctions are well known to those skilled in the art and it is therefore not intended to describe them further.
- the lasing threshold of the lasing portion 133 When the current applied to the electrical contact 155 reaches a certain level the lasing threshold of the lasing portion 133 is reached. Once the lasing threshold is reached the round-trip gain equals the round-trip losses for the lasing cavity 130 . It will be appreciated by those skilled in the art that the gain of the semiconductor active medium 107 in the lasing portion 133 is clamped to the gain value required to offset the round-trip losses and consequently the carrier number is clamped. The optical signal 102 is amplified according to this gain value in the lasing portion 133 of the amplification path.
- the non-lasing portion 135 of the amplification path does not include the lasing cavity 130 no lasing occurs in the non-lasing portion 135 of the amplification path.
- the gain of the semiconductor active medium 107 in the non-lasing portion 135 is unaffected by lasing conditions as the carriers are not clamped to offset round trip losses associated with lasing. It will be appreciated therefore by those skilled in the art that the gain of the non-lasing portion 135 is not clamped to a gain value set by lasing conditions but to the gain characteristics of the semiconductor active medium 107 .
- the potential gain in the non-lasing portion 135 is significantly higher than in the lasing portion 133 .
- the lasing cavity is arranged for clamping a predetermined percentage of the active medium to a predetermined gain value.
- the percentage of the active medium which is clamped may be one of the following ranges 20% to 90%, 30% to 80%, 40% to 70%, 50% to 60%, and 60% to 70% which are given by way of example.
- the active medium 107 of the SOA 100 produces spontaneous emission (SE) which is amplified from the amplification process.
- Amplified spontaneous emission (ASE) is a light matter interaction which is a spurious effect in optical amplification acting like noise.
- the injected signal 102 has a signal-to-noise ratio (S/N). When the signal is injected into the amplification path the noise level increases due to the ASE.
- S/N signal-to-noise ratio
- NF noise figure
- the present inventors provide an amplification path with a lasing portion and a non-lasing portion for reducing the effects of ASE which in turn reduces the noise figure of the SOA 100 .
- the spontaneous emission (SE) generated at the input 110 which propagates towards the output 113 is mostly amplified in the non-lasing portion 135 of the SOA.
- the SE generated at the output 113 which propagates from the output 113 towards the input 110 is amplified less by the lasing portion 133 as the carrier density in this section of the amplification path is clamped by the lasing effects.
- the SE is mostly amplified by the non-lasing portion 135 and not so much by the lasing portion 133 .
- the amplification path having a lasing portion and a non-lasing portion results in carriers in the non-lasing portion of the SOA 100 being consumed mostly by the amplification process of the injected signal and rather than by the spontaneous emission process.
- a reduction of the ASE travelling from the output to the input results in an automatic increase of the carrier density at the input section of the amplification path which reduces the NF of the input section and as a consequence (assuming the output is well designed) lowers the overall NF of the SOA.
- the ratio of amplification of the optical signal with respect to ASE is controlled, such that the optical signal is preferentially amplified with respect to the ASE.
- the graph of FIG. 5 shows the noise figure of the SOA 100 as a function of the percentage of the lasing portion 133 occupying the amplification path (active medium 107 ).
- a number of different bias currents in this case, 150 mA, 200 mA, 250 mA, 300 mA, 350 mA are applied to the electrical contact 155 at different times.
- the noise figure for all bias currents is the lowest when the lasing portion occupies ⁇ 60% of the active medium.
- the graph of FIG. 6 shows the noise figure of the semiconductor optical amplifier 100 as a function of the input power. When the lasing portion 133 occupies ⁇ 20% to 60% of the amplification path results in the lowest noise figure.
- the conventional SOA with no embedded lasing cavity (0% conventional SOA) has a higher noise figure than SOAs with a lasing portion 133 that occupies ⁇ 20% to 60% of the amplification path until the input power reached ⁇ 5 dBm.
- An SOA with a lasing cavity occupying the full length of the amplification path (100% LOA) has the highest noise figure which is substantially independent of the injected power.
- the values of the pump current are given by way of example only and it is not intended to limit the pump currents to these particular values.
- the non-lasing portion 135 has a high carrier density concentration as this region of the amplification path does not experience lasing to consume the carriers.
- the lasing portion 133 has a low carrier density as this region of the amplification path experiences lasing conditions.
- the non-lasing portion defines a region of high carrier concentration, and the lasing portion defines a region of low carrier concentration.
- the carrier concentration gradient along the amplification path defines a maximum occurring in the non-lasing portion and a minimum in the lasing portion.
- NF noise figure
- N carrier density
- the non-lasing portion also defines a region with a relatively low noise figure
- the lasing portion defines a region with a higher noise figure than the non-lasing portion 135 .
- the noise figure of the non-lasing portion 135 is less than the noise figure of the lasing portion 133 .
- the noise figure of the non-lasing portion 135 is substantially equal to the noise figure of the overall SOA 100 .
- the overall NF of the device is given according to the following formula:
- nf T nf 1 + nf 2 - 1 g 1 + nf 3 - 1 g 1 ⁇ g 2 + nf 4 - 1 g 1 ⁇ g 2 ⁇ g 3 + ...
- g is the gain per section
- nf is the noise figure per section.
- the round-trip gain equals the round-trip losses.
- the gain of the semiconductor active medium 107 in the lasing portion 133 is clamped to the gain value required to offset the round-trip losses.
- the optical signal 102 is amplified in the lasing portion 133 according to the clamped gain value resulting from lasing.
- the lasing portion 133 does not contribute significantly to the amplification of ASE.
- the overall level of the ASE associated with the SOA 100 is kept low as amplification of the spontaneous emission ASE associated to the active medium is significantly reduced compared to SOA's known heretofore.
- FIG. 8 there is provided another optical amplifier 200 for amplifying an optical signal.
- the optical amplifier 200 is substantially similar to optical amplifier 100 and like components are indicated by the same reference numerals.
- the main difference between the optical amplifier 200 and the amplifier 100 is that the control means is provided as a resistor network 205 instead of a lasing cavity 130 .
- a plurality of electrical contacts 210 are located along the p-type cladding layer 122 for pumping the active medium with current of varying levels thereby selectively varying the carrier density along the amplification path which in turn selectively controls the amplified spontaneous emission (ASE) associated with the SOA 200 .
- ASE amplified spontaneous emission
- the resistor network 205 divides the amplification path into a plurality of discrete sections s 1 -s 8 each being independently biased with a corresponding one of the pump currents i 1 -i 8 .
- the carrier density is controlled to define a first portion of the amplification path which contributes to a major part of the amplified spontaneous emission (ASE), and a second portion which contributes to a minor part of the amplified spontaneous emission.
- the resistor network 205 is operably coupled to the electrical contacts 210 such that the active medium is pumped with currents of varying levels which progressively decrease from the input 110 to the output 113 .
- the resistance of the resistors R 1 to R 8 are determined by characteristics of the active material 107 and the desired current in each section.
- the resistor network 205 can be provided as discrete resistors or as integrated resistors fabricated on the SOA 200 .
- the resistor network 205 divides the current applied at an input node 215 into plurality of discrete currents i 1 to i 8 which pump corresponding sections s 1 to s 8 of the active medium 107 .
- the resistor network 205 is given by way of example only, it will be appreciated by those skilled in the art that it be may provided with any desired number of resistors or configurations.
- the SOA 100 and the SOA 200 may be provided as signal processing elements in an optical network. Additionally, the SOA 100 and the SOA 200 may be provided as electronic chips. It will be understood that what has been described herein are some exemplary embodiments of a SOA for amplifying an optical signal. Exemplary arrangements include control means being co-operable with the active medium for selectively varying carrier density along the amplification path to improve the signal to noise ratio of the output optical signal. In this way the variance of the carrier density is used to change the ratio of amplification of the optical signal with respect to any background noise such as that contributed from SE.
Abstract
Description
- The present invention relates to a semiconductor optical amplifier (SOA) with a reduced noise figure. The present invention more particularly relates to a semiconductor optical amplifier with a control arrangement for selectively varying the carrier density along the amplification path thereby selectively controlling the amplified spontaneous emission (ASE) and consequently the noise figure.
- Semiconductor Optical Amplifiers (SOA) are essential components in optical networks. Besides acting as optical amplifiers, their inherent non-linearities allow them to form the basis of many signal processing elements (e.g. wavelength converters, logic gates). Compared to the Erbium Doped fibre amplifier (EDFA), Erbium Doped waveguide amplifier (EDWA) or Raman amplifier, the SOA has many advantages: lower cost, larger bandwidth, smaller size, and the potential to be integrated on a chip with electrical pumping. However SOAs suffer from the disadvantage that they have a higher noise figure than EDFA or EDWA amplifiers.
- Attempts have been made to reduce the noise figure of SOAs. One solution involved embedding a lasing cavity inside the SOA. However, this arrangement resulted in a significant reduction of gain because at a certain level of bias current, the SOA starts lasing, this results in clamping of the carrier density at a specific value corresponding to when the gain equals the cavity losses.
- Therefore there is a need for a semiconductor optical amplifier with a reduced noise figure and a relatively high gain.
- These and other problems are addressed by providing a semiconductor optical amplifier with a control arrangement for selectively varying the carrier density along the amplification path.
- Accordingly, a first embodiment of the invention provides a semiconductor optical amplifier as detailed in
claim 1. The invention also relates to a processing element as detailed in claim 42. Additionally, the invention relates to an electronic chip as detailed in claim 43. Advantageous embodiments are provided in the dependent claims. - These and other features will be better understood with reference to the followings Figures which are provided to assist in an understanding of the teaching of the invention.
- The present invention will now be described with reference to the accompanying drawings in which:
-
FIG. 1 is a perspective view of a semiconductor optical amplifier (SOA) in accordance with the present invention. -
FIG. 2 is a cross sectional plan view of the SOA ofFIG. 1 . -
FIG. 3 is a cross sectional side view of the SOA ofFIG. 1 -
FIG. 4 shows the SOA with the active medium extending into the mirrors. -
FIG. 5 shows a graph of the noise figure of the SOA ofFIG. 1 as a function of the percentage of the lasing portion for various values of pumping currents. -
FIG. 6 shows a graph of the noise figure of the SOA ofFIG. 1 as a function of the input power for various values of lasing proportions. -
FIG. 7 shows the carrier density distribution as a function of position along the amplified path of the SOA ofFIG. 1 for various values of pumping currents. -
FIG. 8 is a perspective view of another semiconductor optical amplifier (SOA) in accordance with the present invention. - The invention will now be described with reference to some exemplary semiconductor optical amplifiers (SOA) which are provided to assist in an understanding of the teaching of the invention.
- Referring to the drawings and initially to
FIGS. 1 to 4 there is provided a semiconductor optical amplifier (SOA) 100 for amplifying anoptical signal 102. The present inventors have realised that by providing theSOA 100 with a control arrangement for selectively varying the carrier density along an amplification path significantly reduces the amplified spontaneous emission (ASE) associated with theSOA 100 which in turn reduces the noise figure. The SOA 100 defines alongitudinal axis 105 and atransverse axis 106 which is orthogonal to thelongitudinal axis 105. A semiconductoractive medium 107 is provided on theSOA 100 which defines a single mode amplification path for amplifying theoptical signal 102 as theoptical signal 102 propagates along the amplification path. Theactive medium 107 has substantially uniform dimensions and is arranged to be coaxial with thelongitudinal axis 105. Theactive medium 107 extends between aninput 110 and a spacedapart output 113 which accommodates theoptical signal 102 there through. In this exemplary arrangement, theactive medium 107 is an elongated strip and is encapsulated (surrounded) by cladding 118. Thecladding 118 comprises a p-type cladding layer 122 and an n-typecladding type layer 125 which together encapsulate theactive medium 107. The p-type cladding layer 122 and the n-type cladding layer 125 together with the active medium form an SOA junction. Theactive medium 107 may comprises any suitable amplification material such as but not limited to Quantum wells, Multiple Quantum Wells, bulk material, q-dot, q-dash. Semiconductor active materials are well known to those skilled in the art, and is not intended to describe the active medium further. - In this embodiment the control means comprises a
lasing cavity 130 located on thecladding 118 for providing lateral lasing conditions in the amplification path towards theoutput 113. For illustrative purposes thelasing cavity 130 can be considered to be enclosed by thedashed line 131 inFIG. 3 . Thelasing cavity 130 extends from theoutput 113 and terminates spaced apart from theinput 110. Theactive medium 107 along the length of the lasingcavity 130 forms part of the lasingcavity 130. Thus, thelasing cavity 130 includes the amplification path towards theoutput 113 but not the amplification path towards theinput 110 as thelasing cavity 130 does not extend the full length of theactive medium 107. In this exemplary arrangement thelasing cavity 130 is of length which corresponds to approximately 60% of the length of theactive medium 107, as illustrated best inFIG. 2 . Therefore approximately 40% of the length of the active medium is not included in the lasingcavity 130. The lasingcavity 130 divides the amplification path into two distinct portions, namely, alasing portion 133 which forms part of thelasing cavity 130 and anon-lasing portion 135 which is not part of thelasing cavity 130. The size of the lasing and non-lasing portions may vary depending on the application to which the SOA 100 is applied. The precise values of 40% and 60% are given by way of example only, and it is not intended to limit theSOA 100 to these precise values. The lasingcavity 130 is arranged for facilitating transverse lasing with respect to the direction of theoptical signal 102 as theoptical signal 102 propagates along thelasing portion 133 of the amplification path. When lasing occurs in the lasingcavity 130 it sets the carrier distribution in the lasingportion 133 to a predetermined gain value. It will be appreciated by those skilled in the art that the gain in thenon-lasing portion 135 remains substantially unaffected by lasing conditions in thelasing cavity 130. - The lasing
cavity 130 also comprises a pair of highlyreflective mirrors 140 located on respective opposite sides of theactive medium 107.Lateral cavities 144 are formed on thecladding 118 for accommodating therespective mirrors 140 therein. WhileFIG. 1 shows themirrors 140 of uniform dimensions, in an alternative arrangement, the dimensions of themirrors 140 progressively increase along thelasing portion 133 towards theoutput 113 such that themirrors 140 define a stair case arrangement when viewed in plan view. In this arrangement the steps of themirrors 140 may be provided as discrete segments. Themirrors 140 may be any suitable type such as Bragg reflectors. It is not intended to limit themirrors 140 to any particular configuration or type, Bragg reflectors are given by way of example only. While themirrors 140 have been shown to be spaced apart from theactive medium 107, the present inventors envisage that theactive medium 107 may be extended into themirrors 140 as illustrated inFIG. 4 . Themirrors 140 in the exemplary arrangement consist of air and cladding material. However, it will be appreciated by those skilled in the art that themirrors 140 may be formed from a mix of dielectric materials. - The p-
type cladding layer 122 defines a platform in the form of aridge 150, in this case, of rectangular cross sectional area on which anelectrical contact 155 is supported for facilitating pumping theactive medium 107 with current, as best illustrated inFIG. 1 . Theelectrical contact 155 is substantially the same width as theactive medium 107. Theelectrical contact 155 defines a longitudinal axis which is substantially coaxial with thelongitudinal axis 105 of theSOA 100. Theelectrical contact 155 is in registration with theactive medium 107 and spaced apart therefrom by the P-type cladding layer 122. It is not intended to limit the invention to the pumping arrangement described in the exemplary embodiment. It will be appreciated by those skilled in the art that alternative arrangements may be provided for pumping theactive medium 107 with current. - In operation, the
SOA 100 receives theoptical signal 102 to be amplified at theinput 110 of the amplification path. As theoptical signal 102 propagates along the amplification path the semiconductoractive medium 107 amplifies theoptical signal 102 such that an amplified version of theoptical signal 102 is emitted from theoutput 113 of the amplification path. The semiconductoractive medium 107 is pumped with current by applying electrical current to theelectrical contact 155. The pumping of theactive medium 107 with current causes the P-type cladding layer 122 to inject holes into theactive medium 107, and the N-type cladding layer 125 to inject electrons into theactive medium 107 resulting in theactive medium 107 being pumped with carriers. The operation of SOA junctions are well known to those skilled in the art and it is therefore not intended to describe them further. - When the current applied to the
electrical contact 155 reaches a certain level the lasing threshold of thelasing portion 133 is reached. Once the lasing threshold is reached the round-trip gain equals the round-trip losses for thelasing cavity 130. It will be appreciated by those skilled in the art that the gain of the semiconductor active medium 107 in thelasing portion 133 is clamped to the gain value required to offset the round-trip losses and consequently the carrier number is clamped. Theoptical signal 102 is amplified according to this gain value in thelasing portion 133 of the amplification path. However, as thenon-lasing portion 135 of the amplification path does not include thelasing cavity 130 no lasing occurs in thenon-lasing portion 135 of the amplification path. Thus, the gain of the semiconductor active medium 107 in thenon-lasing portion 135 is unaffected by lasing conditions as the carriers are not clamped to offset round trip losses associated with lasing. It will be appreciated therefore by those skilled in the art that the gain of thenon-lasing portion 135 is not clamped to a gain value set by lasing conditions but to the gain characteristics of the semiconductoractive medium 107. Thus, the potential gain in thenon-lasing portion 135 is significantly higher than in thelasing portion 133. The lasing cavity is arranged for clamping a predetermined percentage of the active medium to a predetermined gain value. The percentage of the active medium which is clamped may be one of the followingranges 20% to 90%, 30% to 80%, 40% to 70%, 50% to 60%, and 60% to 70% which are given by way of example. - The
active medium 107 of theSOA 100 produces spontaneous emission (SE) which is amplified from the amplification process. Amplified spontaneous emission (ASE) is a light matter interaction which is a spurious effect in optical amplification acting like noise. The injectedsignal 102 has a signal-to-noise ratio (S/N). When the signal is injected into the amplification path the noise level increases due to the ASE. To quantify the reduction of the S/N the noise figure (NF) is used. It corresponds to the ratio of the S/N at the input to the S/N at the output. This ratio is typically greater than 1. - The present inventors provide an amplification path with a lasing portion and a non-lasing portion for reducing the effects of ASE which in turn reduces the noise figure of the
SOA 100. The spontaneous emission (SE) generated at theinput 110 which propagates towards theoutput 113 is mostly amplified in thenon-lasing portion 135 of the SOA. The SE generated at theoutput 113 which propagates from theoutput 113 towards theinput 110 is amplified less by thelasing portion 133 as the carrier density in this section of the amplification path is clamped by the lasing effects. The SE is mostly amplified by thenon-lasing portion 135 and not so much by thelasing portion 133. As a consequence of the amplification path having a lasing portion and a non-lasing portion results in carriers in the non-lasing portion of theSOA 100 being consumed mostly by the amplification process of the injected signal and rather than by the spontaneous emission process. A reduction of the ASE travelling from the output to the input results in an automatic increase of the carrier density at the input section of the amplification path which reduces the NF of the input section and as a consequence (assuming the output is well designed) lowers the overall NF of the SOA. In this way the ratio of amplification of the optical signal with respect to ASE is controlled, such that the optical signal is preferentially amplified with respect to the ASE. - Referring now to the graphs of
FIGS. 5 to 7 , the graph ofFIG. 5 shows the noise figure of theSOA 100 as a function of the percentage of thelasing portion 133 occupying the amplification path (active medium 107). A number of different bias currents, in this case, 150 mA, 200 mA, 250 mA, 300 mA, 350 mA are applied to theelectrical contact 155 at different times. The noise figure for all bias currents is the lowest when the lasing portion occupies ˜60% of the active medium. The graph ofFIG. 6 shows the noise figure of the semiconductoroptical amplifier 100 as a function of the input power. When thelasing portion 133 occupies ˜20% to 60% of the amplification path results in the lowest noise figure. The conventional SOA with no embedded lasing cavity (0% conventional SOA) has a higher noise figure than SOAs with alasing portion 133 that occupies ˜20% to 60% of the amplification path until the input power reached ˜−5 dBm. An SOA with a lasing cavity occupying the full length of the amplification path (100% LOA) has the highest noise figure which is substantially independent of the injected power. The values of the pump current are given by way of example only and it is not intended to limit the pump currents to these particular values. - Referring now to the graph of
FIG. 7 which shows the carrier density distribution along the amplification path of theSOA 100 for three values of bias current, in this case, 150 mA, 250 mA and 350 mA. Thenon-lasing portion 135 has a high carrier density concentration as this region of the amplification path does not experience lasing to consume the carriers. Thelasing portion 133 has a low carrier density as this region of the amplification path experiences lasing conditions. Thus, the non-lasing portion defines a region of high carrier concentration, and the lasing portion defines a region of low carrier concentration. The carrier concentration gradient along the amplification path defines a maximum occurring in the non-lasing portion and a minimum in the lasing portion. Various values of pumping current may be applied for shifting the maximum to a desirable value while the minimum remains substantially unaffected by the pumping current. As the noise figure (NF) is inversely proportional to carrier density (N), the non-lasing portion also defines a region with a relatively low noise figure, and the lasing portion defines a region with a higher noise figure than thenon-lasing portion 135. Thus, the noise figure of thenon-lasing portion 135 is less than the noise figure of thelasing portion 133. The noise figure of thenon-lasing portion 135 is substantially equal to the noise figure of theoverall SOA 100. The overall NF of the device is given according to the following formula: -
- g is the gain per section, and
- nf is the noise figure per section.
- When the lasing threshold of the
lasing cavity 130 is reached, the round-trip gain equals the round-trip losses. The gain of the semiconductor active medium 107 in thelasing portion 133 is clamped to the gain value required to offset the round-trip losses. Theoptical signal 102 is amplified in thelasing portion 133 according to the clamped gain value resulting from lasing. When the combined ASE propagates frominput 110 tooutput 113, only thenon-lasing portion 135 of theSOA 100 contributes significantly to the amplification of its level. When ASE2 propagates fromoutput 113 to input 110, the carrier density in thelasing portion 133 is clamped by the lasing effect. Thus, thelasing portion 133 does not contribute significantly to the amplification of ASE. The overall level of the ASE associated with theSOA 100 is kept low as amplification of the spontaneous emission ASE associated to the active medium is significantly reduced compared to SOA's known heretofore. - Referring now to
FIG. 8 there is provided anotheroptical amplifier 200 for amplifying an optical signal. Theoptical amplifier 200 is substantially similar tooptical amplifier 100 and like components are indicated by the same reference numerals. The main difference between theoptical amplifier 200 and theamplifier 100 is that the control means is provided as aresistor network 205 instead of alasing cavity 130. A plurality ofelectrical contacts 210 are located along the p-type cladding layer 122 for pumping the active medium with current of varying levels thereby selectively varying the carrier density along the amplification path which in turn selectively controls the amplified spontaneous emission (ASE) associated with theSOA 200. Theresistor network 205 divides the amplification path into a plurality of discrete sections s1-s8 each being independently biased with a corresponding one of the pump currents i1-i8. In this exemplary arrangement, the carrier density is controlled to define a first portion of the amplification path which contributes to a major part of the amplified spontaneous emission (ASE), and a second portion which contributes to a minor part of the amplified spontaneous emission. Theresistor network 205 is operably coupled to theelectrical contacts 210 such that the active medium is pumped with currents of varying levels which progressively decrease from theinput 110 to theoutput 113. The resistance of the resistors R1 to R8 are determined by characteristics of theactive material 107 and the desired current in each section. Theresistor network 205 can be provided as discrete resistors or as integrated resistors fabricated on theSOA 200. Theresistor network 205 divides the current applied at aninput node 215 into plurality of discrete currents i1 to i8 which pump corresponding sections s1 to s8 of theactive medium 107. Theresistor network 205 is given by way of example only, it will be appreciated by those skilled in the art that it be may provided with any desired number of resistors or configurations. - The
SOA 100 and theSOA 200 may be provided as signal processing elements in an optical network. Additionally, theSOA 100 and theSOA 200 may be provided as electronic chips. It will be understood that what has been described herein are some exemplary embodiments of a SOA for amplifying an optical signal. Exemplary arrangements include control means being co-operable with the active medium for selectively varying carrier density along the amplification path to improve the signal to noise ratio of the output optical signal. In this way the variance of the carrier density is used to change the ratio of amplification of the optical signal with respect to any background noise such as that contributed from SE. While the present invention has been described with reference to some exemplary arrangements it will be understood that it is not intended to limit the teaching of the present invention to such arrangements as modifications can be made without departing from the spirit and scope of the present invention. In this way it will be understood that the invention is to be limited only insofar as is deemed necessary in the light of the appended claims. - Similarly the words comprises/comprising when used in the specification are used to specify the presence of stated features, integers, steps or components but do not preclude the presence or addition of one or more additional features, integers, steps, components or groups thereof.
Claims (45)
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FR2986916A1 (en) * | 2012-02-09 | 2013-08-16 | Eolite Systems | OPTICAL AMPLIFIER AND PULSE LASER SYSTEM WITH IMPULSE ENERGY LIMITS. |
KR20140092214A (en) * | 2013-01-15 | 2014-07-23 | 오므론 가부시키가이샤 | Laser oscillator |
US11837838B1 (en) | 2020-01-31 | 2023-12-05 | Freedom Photonics Llc | Laser having tapered region |
US20230023686A1 (en) * | 2021-06-10 | 2023-01-26 | Freedom Photonics Llc | Designs for lateral current control in optical amplifiers and lasers |
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GB2465754B (en) | 2011-02-09 |
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